This invention involves improvements to delay compensation systems and methods in a wireless discrete tone communications system.
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing good service to a large population of users. Wireless systems consist of cells, each of which include a base station and remote units. The Personal Wireless Access Network described in the Alamouti, et al patent application cited above, is an example of a successful technology for wireless service.
The personal wireless access network (PWAN) system described in the referenced Alamouti et al. patent application, uses a form of protocol known as discrete tone to provide efficient communications between a base station and a plurality of remote units. In this protocol, the user""s data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spatial spreading codes that distribute the data signals over many discrete tones covering a broad range of frequencies or tones. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears a different data signal. The weighted tone set for a particular user is transmitted to the receiving station where it is processed with spatial despreading codes to recover the user""s data signal. For each of the spatially separated antennas at the receiver, the received discrete tone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of the despreading weights are combined with the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular discrete tone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHz of available bandwidth in the range of 1850 to 1990 MHz. The spacing between the tones is 3.125 kHz. The total set of tones are numbered consecutively from 0 to 2559 starting from the lowest frequency tone. The tones are used to carry traffic messages and overhead messages between the base station and the plurality of remote units. The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones.
In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and the remote units. A Common Link Channel (CLC) is used by the base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from the Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with the base station.
In the PWAN system, Frequency Division Duplexing, (FDD) is used by the base station and the remote unit to transmit data and control information in both directions. Transmission from the base station to the remote unit is called forward transmission and transmission from the remote unit to the base station is called reverse transmission. The base station and each remote unit must synchronize and conform to the timing structure and both the base station and the remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it must acquire synchronization from the base station so that it can exchange control and traffic messages within the prescribed time format. The remote unit must also acquire phase synchronization for the signals so that the remote is operating at the same frequency and phase as the base station.
When a remote unit is first installed, it transmits a signal over the CAC channel to the base station. This signal will probably be received at the base station at a time which is not the same as the other remote units transmitting to the base station. The difference between the expected time of the signal, and the time that the signal actually arrives at the base station, is the delay.
Previous systems that compensate for this delay have included systems which have a delay time measurement resolution that is limited. Furthermore, delay time measurement in a protocol is subject to both noise and multipath fading.
This invention solves the delay compensation problem by providing the base station with the ability to measure the magnitude and angle of the phase of delay compensation tones transmitted by the remote unit (RU) on the Common Access Channel (CAC), and calculating the slope of the phase ramp across the frequency range. Because the delay is proportional to the slope of the phase ramp, the delay can be calculated and transmitted to the RU on the Common Link Channel. The RU is provided with the ability to adjust the timing of the signals sent from the RU to the base station on the CAC.
The delay compensation pilots are discrete tones with frequencies that are uniformly spaced throughout each of the upper and lower sub-band frequency ranges. These tones are transmitted by the RU on the CAC and received by the base station. The received tones are digitized, sampled, passed through a Fast Fourier Transform (FFT) processor and stored in FFT incremental frequency bins as complex numbers. These numbers represent points in a 16 Quadrature Amplitude Modulation (QAM) constellation and are related to the average amount of energy of the received tone in the increment of frequency represented by the FFT bin.
In accordance with this invention, the base station uses the information about the signals received, as represented by the FFT bins, to calculate the amount of delay and prepare a signal containing the corresponding amount of compensation to be sent to the RU on the CLC.
In accordance with the invention, the phase difference between the expected phase angle and the received phase angle is calculated for each delay compensation pilot (DCP) tone. The difference in phase between each successive DCP tone is proportional to the time delay and the uniform frequency difference between tones. Because of this, the plot of phase difference as a function of frequency is a constant slope line, where the slope is proportional to the time delay.
In a first embodiment of the invention, the phase angle differences are measured by taking the FFT output representing the first DCP and multiplying it by the complex conjugate of the all sixteen of the FFT outputs from one of the two sub-bands. This gives sixteen phase angle measurements for slope calculation.
In a second embodiment of the invention, each symbol corresponding to the 16 elements of the QAM constellation is correlated with each symbol sequence from the sixteen FFT outputs from one sub-band. This allows sixteen phase-angle measurements based on the correlation coefficients.
In a third embodiment of the invention, the output of the FFT from both sub-bands is used. Sixteen symbol sequences are created from each sub-band, and then the first sequence of each sub-band are added to each other, then the second sequence of each sub-band, and so on until the sixteenth sequence of each sub-band are added together. Sixteen phase angle measurements are generated when the QAM symbol sequence is correlated with each of the 16 symbol sequences.
In a fourth embodiment of the invention, the output of the FFT from both sub-bands and from eight antenna elements is used. Sixteen symbol sequences are received from each sub-band for each antenna element. Sixteen symbol sequences are created when the 32 first sequences of each sub-band for each antenna element are summed, then the 32 second sequences of each sub-band for each antenna are summed, and so on until the 32 sixteenth sequences of each sub-band for each antenna are summed. Each summed sequence is normalized and correlated with the QAM symbol sequence to generate 16 phase angle measurements.
Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well.